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Tiny Tin Breaks Melting Rules

As any fondue chef knows, shredded cheese melts faster than large chunks. This property also holds for very small crystals: The smaller they are, the faster they melt. Now, surprised researchers report that tiny clusters of 10 to 30 tin atoms violate this rule: They remain solid at temperatures well above bulk tin's melting point.

For more than 100 years, researchers have worked under the assumption that smaller crystals melt easily because they have a larger percentage of atoms on the surface, where they're relatively weakly bound to the structure. Until now, this theory held even for tiny clusters of fewer than about 1000 atoms. To measure the melting points of such small crystals, researchers float them in a gas and watch as the rigid crystals melt into liquid balls.

While experimenting with a new method for measuring the melting point of very small clusters, chemists Martin Jarrold and Alexandre Schvartsburg of Northwestern University in Evanston, Illinois, discovered that the smallest tin clusters are reluctant to melt. The researchers used a weak electric field to drag the cigar-shaped clusters of 10 to 30 tin atoms through a metal drift tube filled with helium. The time it takes a cluster to cross the tube should drop dramatically when the tin melts because spherical droplets move more easily through the gas than crystals. But it never did. The cluster failed to melt even when the researchers raised the temperature to 282°C, more than 50 degrees above the melting point of bulk tin, they report in the 18 September issue of Physical Review Letters. A balky apparatus prevented them from reaching temperatures above 282°C.

The researchers speculate that the trigonal structure of the clusters, which differs from bulk tin's tetragonal shape, somehow prevents it from melting. But while physicist David Wales of Cambridge University in the United Kingdom agrees that structure is an important clue, he cautions that "we don't know what on earth is going on." Wales predicts the finding will "stimulate a lot of theoretical and experimental work."